Thompson Lake is one of approximately 26 water bodies in Maine that have documented infestations of variable leaf milfoil. Variable leaf milfoil, an invasive species, is an aggressive plant, which, by forming dense mats of vegetation, crowds out native plants. Thick growth of this plant can ruin boating, swimming, and other recreational uses of a water body. The plant spreads mainly by fragmentation.

Maine has six species of native milfoil, but it is the invasive variable leaf milfoil that causes most concern. It is most prevalent in shallower water up to 15 feet deep. While positive identification of the plant is not easy and should be left to the experts, variable leaf milfoil has some distinguishing characteristics. Its long stems with whorls of finely divided leaves are rooted in the bottom and rise vertically to the surface of the water. Near the surface, especially in shallower water, stems are typically red and may turn and grow horizontally, often forming thick mats. The plant has two distinct types of leaves. The submerged leaves are feather or comb-like, occur in whorls of four to six, and are one to two inches long. The flowers are small and reddish and form on the stem just above the emergent leaves. Download the "Field Guide of Invasive Aquatic Plants" published by Lake Stewards of Maine (formerly Maine Volunteer Lakes Monitoring Program)>


Variable leaf milfoil first appeared in the southern section of Thompson Lake about 1980. At first confined to the part of the lake known as the Heath, it apparently spread into the main lake after the reconstruction of the causeway and culverts that connect the two parts of the lake.


By 2005 the milfoil had spread from one end of the lake to the other, with the heaviest concentrations found in shallow areas, especially in the Heath, the Greeley Brook area, Otisfield Cove, Edwards Cove, and Parson's Point Cove (near Hancock Beach). Once established, milfoil is difficult to control. After consulting with Maine's Department of Environmental Protection, TLEA's Environmental Committee, chaired by Scott Bernardy, decided to try eradicating the milfoil in limited areas by using benthic barriers. These essentially are heavy, opaque plastic tarps placed over the milfoil, and which remain in place for about two months. The effect is to smother the plants. Late in the summer of 2005, volunteers placed eight benthic barriers over the large milfoil infestation in Edwards Cove. Each of the ten weighted tarps covered about 800 square feet of lake bottom. Volunteers also placed benthic barriers with considerable success in Otisfield Cove.

After three years' experience with the barriers, TLEA concluded that the removal work, while successful, had certain limitations. First, this system required the coordination of a large number of volunteers to place and remove the tarpaulins. Observers noted that the method was suitable mainly for small areas of infestation, and this method did not get rid of individual plants growing around docks and other hard-to-reach spots. The other limitation of the benthic barriers was that it is a relatively slow process. The fear was that, using this method alone, the milfoil would grow faster than anyone could remove it.

In the summer of 2007, then, with funds contributed by property owners, TLEA obtained permits and instruction from the DEP to hire and supervise divers to hand pull the milfoil in the Edwards Cove area in Otisfield. The two divers employed succeeded in removing several truckloads of milfoil, a substantial increase in volume.

The divers were expensive, and the amount of milfoil the divers could remove was limited severely by the time-consuming process of handling the plants, that is, the time it took to safely remove the plants from the water. Other lake associations, such as the association working in Little Sebago Lake, had been experimenting with using a pontoon boat as a kind of milfoil-collecting barge.


In the spring of 2008, TLEA directors purchased a second-hand pontoon boat and then proceeded to turn it into a milfoil harvester, given the name "Hippobottomus." At the center of the harvester system is a vacuum pump mounted on the boat. A diver in the water hand pulls the milfoil and then uses a four-inch vacuum hose to deliver the milfoil to a sluice box, also mounted on the boat. The sluice box separates the plant material from the water and packages it into fifty-pound mesh bags. TLEA's boat was modeled on the one the Lakes Environmental Association was using on Little Sebago Lake.


Early in 2010 TLEA received a large grant for milfoil eradication work from the Maine Milfoil Initiative. In order to take full advantage of this federal funding, TLEA decided to double its summer schedule to engage two crews for a total of 12 hours a day. Rob McVety of Otisfield was contracted with TLEA to coordinate the actual milfoil removal.

For the nexr two summers TLEA focused on work conducted aboard the Hippobottomus, which launches divers into the shallow coves to hand pull the variable leaf milfoil. This summer McVety’s 3-man crew is also stepping up efforts to smother the invasive plants with benthic barriers. The new grant funds have made it possible to buy the materials for these barriers, which cost $60 each to construct. To date McVety has constructed 60 new 10 x 30’ barriers, made out of precut tarps, rebar, clear duct tape, and zip ties.

Shortly after the summer’s work began, McVety reported progress being made on all fronts. Hancock Beach “is looking significantly better than last year, with much less milfoil growth visible.” Serenity Cove in Poland was also greatly improved. In most of Edwards Cove the crew found no large colonies. Otisfield Cove was also looking better. The most challenging area for the crew remains Pine Point, where McVety has been laying most of the benthic barriers. He warns boaters leaving this area in particular to check their propellers for hitchhiking plants which they must remove before heading out into the main part of the lake.

"Variable Leaf Milfoil - Thompson Lake"

" The Hippo, photo by Kathy Cain"

"Launch of the Hippobottomus, Photo by Bob Tracy".

"Diver Rob McVety at Edwards Cove, Photo by Scott Bernardy"

"Benthic Barrier Being Placed In Otisfield Cove"

"Retrieving Milfoil From Edwards Cove, Photo by Bot Tracy"

"Bagging Milfoil on the Hippobottomus, Photo by Kathy Cain".

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